This application relates generally to the field of metal casting and more particularly to methods for effectively cooling down casting materials.
Complex castings made of light metals, such as aluminum, typically face a number of heat related challenges that can adversely affect their quality. Two of these challenges are surface tears and near-surface porosity (voids). These casting challenges are related to the heat flow rate and total amount of heat that can be transferred from the casting material into their mold surfaces.
Surface tears typically develop when the temperature of the mold (e.g. steel) surface in contact with the molten casting increases. The increased temperature causes chemical dissolution of the mold surface with the molten casting. Upon casting solidification, parts of the mold surface may bond with the solid casting. This bond makes extraction of the casting difficult, which causes surface tears from the stress of extraction.
Porosity or voids in the casting occur due to metal shrinkage. For example, aluminum casting has a shrink rate of 5% in the molten state and 5% in solid state. Between the molten state and the solid state (i.e., during the solidification process), aluminum shrinks, forming porosity voids. These voids are formed in the region that solidifies last.
In complex castings, some casting regions might be thicker than others, and these areas solidify last. Moreover, to form holes in aluminum castings, “core pins” (solid cylindrical mold sections) are most commonly used. Molten aluminum is poured or injected around these pins and then solidified. In general, core pins absorb large amount of heat from the surrounding casting and are not able to expend this heat anywhere, making these mold elements one of the hottest regions in a mold. The aluminum casting in contact with the core pin solidifies last, causing near surface voids. These voids are usually exposed after the external cast surface is removed from machining.
To prevent surface tears, large amount of heat should be extracted from selected heavy cross-sectional areas of the casting. Similarly, to prevent near-surface porosity voids, a high rate of heat extraction should be obtained during solidification. By extracting a higher quantity of heat from the casting, the final solidification region can be pushed deeper into the casting, allowing formation of any potential shrinkage porosity deeper into the casting.
To combat these mechanical defects, a number of methods have been utilized in the past. In one such method, casters identify the highest temperature points in the mold using infra red heat detectors, and directly spray water on those regions. Although, this method brings down the mold temperature instantaneously, it may substantially harm the metal. Such a large temperature flux (ambient temperature of water is about 40 F and the temperature of hot steel is about 800 F) causes thermal stress, which over time develops into thermal fatigue, reducing the mold life considerably.
Another commonly used method places water lines ¾ of an inch away from the surface of the mold. This distance ensures that the heat flux is not too high at the mold-water interface. Water, however, does not conduct heat efficiently at this distance, resulting in ineffective cooling of the mold. A third method forces brief jets of water through the mold when the mold is under the highest heat load. Subsequently, an air circuit blows the water away. The water vaporizes immediately as it absorbs heat, this hot vapor is sucked out of the mold leaving it relatively cooler. This method is successful for small, inexpensive molds or core pins but cannot be used with complex, expensive casting molds.
Therefore, there exists a need for a device and method to cool castings and die molds (including core pins) effectively and to keep the temperature at the surface of the core pin relatively low in order to avoid near surface porosity and surface tears.